1. Field of the Invention
The field of art to which this invention pertains is the solid-bed adsorptive separation of monosaccharides. More specifically the invention relates to a process for separating glucose from a mixture comprising glucose and fructose which process employs an adsorbent comprising a crystalline aluminosilicate which selectively adsorbs glucose from the feed mixture.
2. Description of the Prior Art
It is well-known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbon types from mixtures thereof. As a few examples, a separation process disclosed in U.S. Pat. Nos. 2,985,589 and 3,201,491 uses a type A zeolite to separate normal paraffins from branched-chain paraffins and processes described in U.S. Pat. Nos. 3,265,750 and 3,510,423 use type X or type Y zeolites to separate olefinic hydrocarbons from paraffinic hydrocarbons. In addition to their use in processes for separating hydrocarbon types, X and Y zeolites have been employed in processes to separate individual hydrocarbon isomers. As a few examples, adsorbents comprising X and Y zeolites are used in the process described in U.S. Pat. No. 3,114,782 to separate alkyl-trisubstituted benzene isomers; in the process described in U.S. Pat. No. 3,864,416 to separate alkyl-tetrasubstituted monocyclic aromatic isomers; in the process described in U.S. Pat. No. 3,668,267 to separate specific alkyl-substituted naphthalenes. Because of the commercial importance of para-xylene, perhaps the more well-known and extensively used hydrocarbon isomer separation processes are those for separating para-xylene from a mixture of C.sub.8 aromatics. In processes described in U.S. Pat. Nos. 3,558,730; 3,558,732; 3,626,020; 3,663,638; and 3,734,974 for example adsorbents comprising particular zeolites are used to separate para-xylene from feed mixtures comprising para-xylene and at least one other xylene isomer by selectively adsorbing para-xylene over the other xylene isomers.
In contrast, our invention relates to the separation of non-hydrocarbons and more specifically to the separation of monosaccharides. We have surprisingly discovered that an adsorbent comprising an X zeolite containing potassium cations at the exchangeable cationic sites uniquely exhibits adsorptive selectivity for an aldose with respect to a ketose, particularly glucose with respect to fructose, thereby making separation of glucose from a mixture comprising glucose and fructose by solid-bed selective adsorption possible. Glucose (dextrose) is widely used in confectionery and baking industries, in canning of fruits and vegetables, in beverages and other products requiring sweeteners, and for the preparation of caramel color. In some instances it is used directly as replacement, wholly or in part, for cane or beet sugar; in other instances the special properties of glucose are utilized. While highpurity glucose is readily manufactured from starch (which is made up exclusively of glucose units) by hydrolysis with mineral acids at elevated temperature followed by refining and crystallization of the hydrolyzate, this embodiment of our process is primarily intended for separating glucose from a feed mixture such as an invert sugar solution containing both glucose and fructose to obtain two product streams; an extract product stream containing glucose in a higher concentration than that found in the feed mixture and a raffinate product stream containing fructose in a higher concentration than that found in the feed mixture. Both products can be employed in many commercial uses requiring sweeteners. The raffinate product stream, containing a high concentration of fructose, is of particular commercial interest since fructose is considered to be the most soluble and the sweetest of the sugars. Relative to sucrose having a sweetness of 1.0, fructose has a relative sweetness of about 1.4 while that of glucose is 0.7.